Phase lock loop circuit using optical correlation detection

ABSTRACT

It is an object of the present invention to provide an optical clock phase lock loop capable of a higher operational speed than the conventional technology by utilizing high-speed optical phenomena other than gain modulation. In an optical clock phase lock loop for conducting locking control by means of obtaining the correlation of an optical signal and optical clock, after combining an optical clock of a short pulse containing a harmonics component with an optical signal by means of an optical coupler 102, and providing the resultant light to traveling-wave semiconductor laser amplifier 103, the correlation signal of the optical signal and optical clock included in the output signal of this traveling-wave semiconductor laser amplifier 103 is detected by means of optical bandpass filter 104.

This is a division of application Ser. No. 08/389,486, filed Feb. 16,1995 now U.S. Pat. No. 5,574,588.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical clock phase lock loopcircuit and an optical correlation detection circuit which is ideal forconstructing this optical clock phase lock loop circuit, both of whichare used for locking the frequency and phase of the clock in opticalrepeaters, optical terminal units, and optical signal processing ofultra-high-speed optical transmissions.

2. Relevant Art

FIG. 10 is a diagram showing a structural example of a conventionaloptical clock phase lock loop circuit. In this Figure, an optical signalinput terminal 601, optical coupler 602, traveling-wave semiconductorlaser amplifier 603, optical bandpass filter 604, optical receivingcircuit 605, phase comparator 606, voltage-controlled oscillator 607(hereinafter referred to as "VCO"), micro-wave mixer 608, optical pulsegenerator 609, optical pulse multiplexer 610, low frequency transmitter611, and frequency multiplier 612 are provided. The oscillationfrequency of VCO 607 is denoted by f₀, and the value of this oscillationfrequency f₀ is set such that the bit rate of the optical signalinputted from optical signal input terminal 601 becomes nf₀ (n is aninteger of 1 or greater).

In the following, a concrete construction of the optical pulsemultiplexer will be explained. FIG. 11 is a structural diagram of 2×optical pulse multiplexer using an optical fiber. In this Figure, inputterminal 701, optical fiber coupler 702, optical fiber delay line 703,optical fiber coupler 704, and output terminal 705 are provided. In thisstructure, the clock optical pulse entering from input terminal 701 isdivided into two by means of optical fiber coupler 702. One of thesedivided clock optical pulses is delayed by T/2+mT (T is the time slot ofthe input clock optical pulse=1/f₀, and m is an integer) by means ofoptical fiber delay line 703 and then coupled again by means of opticalfiber coupler 704 such that a repetition frequency of 2× (multiplied bytwo) optical clock is formed. In the above case, the repetitionfrequency of the optical clock formed becomes 2× (f₀ +Δf). In the casewhen the multiplicity of the clock is greater than 2, the aforementionedmultiplexer is connected in a multistage manner. The multiplicity of theclock then becomes 2^(K) by means of a connection of k stages. However,in this case, the delay amount of the optical fiber delay line used inthe kth multiplexer from the incident side is (T/2^(K) +mT).

FIG. 12 is a structural diagram showing a three-stage 8× optical pulsemultiplexer using an optical wave guide. In this Figure, an inputterminal 801, optical combining/splitting device 802, optical waveguides803 and 804, optical splitter 805, optical waveguides 806 and 807,optical combining/splitting device 808, optical waveguides 809 and 810,optical combining/splitting device 811, and output terminal 812 areprovided. This circuit is constructed by means of integrating thefunctions explained using FIG. 11 onto a silicon substrate. Thefunctions are the same as those of the structure shown in FIG. 11;however, due to monolithic integration, the present circuit exhibits acompact nature and stable operation not affected by fluctuations of thetemperature and the like. An example in which optical pulse multiplexingis achieved using the present circuit is described in S. Kawanishi, etal. ("100 Gbit/s, 50 km, and Non-Repeated Optical Transmission EmployingAll-Optical Multi/Demultiplexing and PLL Timing", Electronics Letters,vol. 29, pp. 1075-1076, 1993).

In the following, the actions of a conventional optical clock phase lockloop circuit shown in FIG. 10 will be explained. The output signal ofVCO 607 undergoes a frequency shift to (f₀ +Δf) by means of lowfrequency oscillator 611 and microwave mixer 608. Optical pulsegenerator 609 is then driven by means of the signal undergoing the abovefrequency shift. As a result, an optical clock in which the repetitionfrequency is (f₀ +Δf) is generated from optical pulse generator 609. Theoptical clock generated from optical pulse generator 609 as describedabove is multiplexed by means of optical pulse multiplexer 610 shown bymeans of either of the structures in FIG. 11 or FIG. 12 and is outputtedas a multiplexed clock in which the repetition frequency is n× (n is anatural number). The optical clock multiplexed by means of optical clockpulse multiplexer 610 is combined with an optical signal pulse fromoptical signal input terminal 601 by means of passing through opticalcoupler 602, and then enters traveling-wave semiconductor laseramplifier 603. At this time, when n=2^(K) is designated in order for thefrequency of the clock following multiplexing to reach a frequency 2^(K)(f₀ +Δf) corresponding to the bit rate nf₀ of the optical signal, evenwhen the optical signal pulse is a completely random modulation signal,the nΔf component, which serves as the correlation of the optical signalpulse and optical clock, is generated by means of these two lights.

In the following, the actions of traveling-wave semiconductor laseramplifier 603 which serves as an optical modulation device will beexplained. The wavelength λ_(sig) of the optical signal entering thetraveling-wave semiconductor laser amplifier 603 and the wavelengthλ_(clk) of the optical clock are apart from each other to a degree suchthat coherent interference is not generated. In this case, when anoptical clock possessing an optical intensity of a certain degree entersthe traveling-wave semiconductor laser amplifier, the carrier withintraveling-wave semiconductor laser amplifier 603 is modulated.Modulation of this carrier means that the gain of traveling-wavesemiconductor laser amplifier 603 is modulated with respect to anoptical signal which serves as another optical input. The principles ofthis modulation are described in detail in Kawanishi, S. et al.("Ultra-high-speed PLL-type clock recovery circuit based on all-opticalgain modulation in traveling-wave laser diode amplifier"; IEEE Journalof Light Wave Technology, vol. 11, pp. 2123-2129; 1993).

As described above, the correlation component of both lights is includedin the optical signal wherein the gain is modulated by means of theabove optical clock. As a result, this optical signal is extracted bymeans of optical bandpass filter 604, converted into an electricalsignal by means of optical receiving circuit 605, and compared with astandard signal by means of phase comparator 606. PLL operation is thenachieved by means of conducting feedback of this output to VCO 607.

In the following, the principle operation of this PLL is explained.Initially, the optical signal (repetition frequency nf₀) inputted fromoptical signal input terminal 601 is combined with a multiplexed opticalclock via optical coupler 602, and inputted into traveling-wavesemiconductor laser amplifier 603. For the sake of simplicity, theoptical signal pulse and clock are both sine waves, and are respectivelyexpressed by Ps(t) and Pc(t) in the formulae below.

    Ps (t)=Ps{1+sin n (2πf.sub.0 t+φ(t))}               (1)

    Ps (t)=Ps{1+sin 2πn (f.sub.0 +Δf) t)}             (2)

In the formulae, Ps and Pc are constants. In addition, φ(t) representsthe phase difference (the reciprocal time difference of pulse positions)between the optical signal pulse and the optical clock. This φ(t)represents the control objective of PLL, and should be set to 0 or aconstant value.

Hence, the optical signal and optical clock enter traveling-wavesemiconductor laser amplifier 603 and undergo gain modulation. At thistime, among the light outputted from traveling-wave semiconductor laseramplifier 603, P_(sout) and P_(cout) correspond to the aforementionedoptical signal and optical clock, respectively. P_(sout) and P_(cout)are expressed by the following formulae.

    P.sub.sout =G·Ps  1+sin n {2πf.sub.0 t+φ(t)}!· 1+m(Pc) sin {2π(f.sub.0 +Δf) t+π}!(3)

    P.sub.cout =G·Pc  1+sin 2nπ(f.sub.0 +Δf)}· 1+m(Ps) sin n {2πf.sub.0 t+φ(t)+π}!(4)

In the above formulae, G represents the unsaturated gain oftraveling-wave semiconductor laser amplifier 603, and m(Pc) and m(Ps)represent the degree of gain modulation from the optical clock andoptical signal, respectively.

At the time when the clock optical pulse intensity reaches the peak, thenumber of carriers within traveling-wave semiconductor laser amplifier603 is minimized. Consequently, the phase of the gain modulation withinthe traveling-wave semiconductor laser amplifier 603 differs from thephase of the incident optical clock by π. Hence, this phase difference πis added as shown in formulae (3) and (4). Among the optical signal andoptical clock outputted from the gain modulated traveling-wavesemiconductor laser amplifier 603, only the optical signal is extractedby means of optical filter 604. Since the gain band width (wavelengthconversion) of traveling-wave semiconductor laser amplifier 603 isapproximately 50 nm, it is possible to extract only one light by meansof optical filter 604 by sufficiently separating the wavelengths of theoptical signal and optical clock within the aforementioned band. Theoptical signal extracted by means of optical filter 604 is inputted intooptical receiving circuit 605. The photocurrent Os(t) at the time ofusing PIN-PD as the optical receiving component of optical receivingcircuit 605 is shown in the following. ##EQU1##

In the above formula, e represents the electron charge, η represents thequantum efficiency of PIN-PD, and hν represents the energy of thephotons. The last item of formula (5) is the nΔf component generated bymeans of correlation with the optical clock, and thus, the fluctuationof the phase difference between the optical signal pulse and the opticalclock pulse is replaced by the fluctuation of this nΔf component. PLLoperation is then accomplished by means of conducting a phase comparisonof this nΔf output and the Δf output of the original oscillator with an-multiplied nΔf reference signal, and then conducting feedback to VCO607.

The actual experimental results can be found in the reference, (S.Kawanishi, et at., "Ultra-high-speed PLL-type clock recover circuitbased on all-optical gain modulation in traveling-wave laser diodeamplifier"; IEEE Journal of Light Wave Technology, vol. 11, pp.2123-2129, 1993). At this point, since the phase comparison is conductedusing the nΔf component, the phase comparison output is in proportion tonΔφ. Consequently, during operation of PLL, when nΔφ is 0, the feedbacksignal to VCO 607 becomes 0, and the power fluctuation of the inputoptical signal is unaffected. By setting nΔf to a value of approximatelyseveral 100 kHz, high-speed operation is not required in the electricalphase comparator 606, and thus, overall, a high-speed PLL operation canbe anticipated.

However, in the above-described conventional PLL circuit, the followingtwo problems exist.

Problem 1

As a method for conducting correlation detection of an optical signalpulse and optical clock pulse, gain modulation of a traveling wavesemiconductor laser amplifier is widely used. However, the modulationefficiency therein depends on the frequency as shown in formulae (3) and(4); more concretely, this efficiency is reduced in a manner inverselyproportional to the square of the frequency. In the calculationdescribed in S. Kawanishi et al. ("Ultra-high-speed PLL-type clockrecover circuit based on all-optical gain modulation in traveling-wavelaser diode amplifier"; IEEE Journal of Light Wave Technology, vol. 11,pp. 2123-2129, 1993), the upper limit of the operational speed of PLLusing gain modulation of a traveling-wave semiconductor laser amplifieris approximately 100 GHz. Achieving an operational speed greater thanthis upper limit is considered to be technically difficult.

Problem 2

As a method for conducting correlation detection of an optical signalpulse and optical clock pulse, a method in which the speed of the clockis multiplied using an optical pulse multiplexer in order to match thespeed of the clock with the bit rate nf₀ of the optical signal pulse isemployed. However, according to this method, not only is the structureof the apparatus involved extremely complex, but also due to theoperational principles of the optical time-division-multiplexingcircuit, the clock frequency is fixed. The variable range of the clockfrequency is at most approximately 5%; any attempts to vary the clockfrequency more than this aforementioned amount results in the generationof jitters (fluctuations) in the clock multiplexing, which in turn leadsto the malfunctioning of the PLL. Furthermore, according to thestructure shown in FIG. 12, only multiples of 2^(K) can be handled,i.e., 2, 4, 8, 16 . . ., and thus, optical clock multiplication ofmultiples of random integers is difficult.

SUMMARY OF THE INVENTION

In consideration of the aforementioned, it is an object of the presentinvention to provide an optical clock phase lock loop circuit capable ofhigh-speed operation, which solves the aforementioned Problem 1 andProblem 2.

Therefore, in order to solve the aforementioned Problem 1, the presentinvention comprises as a method for conducting correlation detection ofan optical signal pulse and optical clock pulse, ultra-high-speed PLLusing a high-speed optical phenomenon faster than that produced by gainmodulation by means of four-wave mixing in traveling-wave semiconductorlaser amplifier, optical fibers and the like, and sum-frequencygeneration in nonlinear optical crystal.

More concretely, the present invention provides an optical clock phaselock loop comprising:

first oscillating means in which an oscillation frequency and phase varyaccording to exterior control;

second oscillating means for outputting an a.c. signal;

frequency mixing means for generating, by means of mixing an outputsignal of said first oscillating means and an output signal of saidsecond oscillating means, a signal of a frequency corresponding to asum, difference, or sum and difference of frequencies of said outputsignals;

optical pulse generating means for generating an optical pulse of arepetition frequency equal to the frequency of an output signal of saidfrequency mixing means;

optical multiplexing means for conducting opticaltime-division-multiplexing of said optical pulse, and for multiplyingsaid repetition frequency by n (n is a natural number);

optical correlation detecting means for outputting the product of lightintensities of an optical signal pulse and an optical clock pulseoutputted from said optical multiplexing means;

optical receiving means for converting an optical output of said opticalcorrelation detecting means into an electrical signal; and

means for controlling a phase of said first oscillating means bycomparing a phase of said electrical signal outputted from said opticalreceiving means and a phase of said signal produced by multiplying saidstandard signal by n, such that said phase difference of said signalsresults in a predetermined value;

wherein, said optical correlation detecting means is formed from aoptical combining means for combining said optical signal pulse and saidoptical clock pulse; traveling-wave semiconductor laser amplifying meansfor amplifying a signal produced by said combination; and opticalsplitting means for extracting from an optical output signal of saidtraveling-wave semiconductor laser amplifying means only the wavelengthof said four-wave mixing light which is a correlation component of saidoptical signal pulse and said optical clock pulse;

said oscillation frequency of said first oscillating means is set to 1/nof the bit rate of said optical signal pulse, and said wavelengths ofsaid optical signal and optical output of said optical pulse generatingmeans are set to values such that said signals coherently interfere witheach other.

According to the aforementioned, correlation detection of an opticalsignal pulse and optical clock pulse is performed using ultrahigh-speedfour-wave mixing phenomenon in the traveling-wave semiconductor laseramplifier, and as a result, even when correlation detection from anultra-high-speed signal is conducted, a clock synchronized to thisultra-high-speed signal can be generated without degrading the mixing ofthe four-wave mixing.

In addition, in order to solve the aforementioned Problem 2, the presentinvention achieves ultra-high-speed optical PLL using as the method fordetecting the correlation between an optical signal pulse and opticalclock pulse, a method for directly detecting a correlation signalbetween a short optical clock pulse and optical signal pulse, whereinthe short optical clock pulse is not subjected totime-division-multiplexing as in the prior art.

More concretely, the optical correlation detecting circuit of opticalPLL comprises:

optical combining means for combining an optical signal pulse and anoptical clock pulse possessing a sufficiently narrow pulse width forincluding nth harmonics (n is a natural number) component of arepetition frequency;

traveling-wave semiconductor laser amplifying means for outputting aproduct of intensities of said optical signal pulse and said opticalclock pulse; and

optical splitting means for extracting from an optical output signal ofsaid traveling-wave semiconductor laser amplifying means only saidcorrelation component of said optical signal pulse and said opticalclock pulse.

In the traveling-wave semiconductor laser amplifying means, gainmodulation from the optical clock pulse, or four-wave mixing between theoptical clock pulse and optical signal pulse is generated to produce anoutput containing a correlation signal between both optical pulses.Subsequently, the correlation signal is detected from this output signalby means of the optical splitting means.

In other words, using a short optical clock pulse which does not undergooptical-time-division-multiplexing, the correlation signal between anoptical signal and an optical clock is detected by means ofultra-high-speed four-wave mixing and the like. As a result, it ispossible to generate a clock synchronized to 1/n of the repetitionfrequency of an ultra-high-speed optical signal. In addition, accordingto the present PLL, it is possible to generate a clock and dividingclock synchronized to a randomly modulated optical signal, and thus thepresent PLL produces superior effects when used in ultra-high-speedoptical transmission, signal processing, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing a structure according to a first embodimentof the present invention.

FIG. 1B is a diagram showing another structure according to a firstembodiment.

FIG. 2A is a diagram showing the spectrum in the case when thewavelengths of the optical signal and optical clock enteringtraveling-wave semiconductor laser amplifier are separated according tothe same embodiment.

FIG. 2B is a diagram showing the case in which the proximity of thewavelengths is to a degree such that both lights coherently interferewith each other.

FIG. 3A is a diagram showing a structure of Modification Example 1according to a first embodiment.

FIG. 3B is a diagram showing another structure of Modification Example1.

FIG. 4A is a diagram showing a structure of Modification Example 2according to a first embodiment.

FIG. 4B is a diagram showing another structure of Modification Example2.

FIG. 5A is a diagram showing a structure of Modification Example 3according to a first embodiment.

FIG. 5B is a diagram showing another structure of Modification Example3.

FIG. 6A is a diagram showing a structure of a second embodimentaccording to the present invention.

FIG. 6B is a diagram showing another structure of a second embodiment.

FIG. 7 is a diagram showing the spectrum of the optical signal and clockpulse inputted into optical coupler 102, and the spectrum of the outputlight of traveling-wave semiconductor laser amplifier 103.

FIG. 8A is a diagram showing a structure of Modification Example 4according to the second embodiment.

FIG. 8B is a diagram showing another structure of Modification Example4.

FIG. 9A is a diagram showing a structure of Modification Example 5according to a second embodiment.

FIG. 9B is a diagram showing another structure of Modification Example5.

FIG. 10 is a diagram showing a conventional structure.

FIG. 11 is a structural diagram showing a 2× multiplexer of an opticalclock using an optical fiber.

FIG. 12 is a structural diagram showing a three-stage 8× optical pulsemultiplexer using an optical wave guide.

FIG. 13 is a diagram showing a structure of Modification Example 6according to a second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the embodiments of the present invention will beexplained with reference to the Figures.

First Embodiment

FIG. 1A is a diagram showing the first embodiment of the presentinvention. In this Figure, optical signal input terminal 101, opticalcoupler 102 traveling-wave semiconductor laser amplifier 103, opticalbandpass filter 104, optical receiving circuit 105, phase comparator106, VCO 107, microwave mixer 108, optical pulse generator 109, opticalpulse multiplexer 110, low frequency oscillator 111, and frequencymultiplier 112 are provided. The oscillation frequency f₀ of VCO 107 isset such that the bit rate of the optical signal inputted from opticalsignal input terminal 101 becomes nf₀ (n is an integer of at least 1),as in the structure shown in FIG. 10 above.

In the following, the operation of the present embodiment will bedescribed.

The output signal of VCO 107 undergoes a frequency shift by means of lowfrequency oscillator 111 and micro-wave mixer 108. Optical pulsegenerator 109 is driven by means of this frequency shifted signal, andthe output pulse of optical pulse generator 109 is multiplexed by meansof optical pulse multiplexer 110 to produce an optical clock in whichthe frequency is multiplexed. The above actions are similar to thosedescribed for the structures shown in FIG. 10.

In the following, the actions of traveling wave semiconductor laseramplifier 103 which serves as the optical modulation means will beexplained. FIGS. 2A and 2B show the relationship between wavelengthλ_(sig) of an optical signal and wavelength λ_(clk) of an optical clockwhich enter a traveling wave semiconductor laser amplifier 103. FIG. 2Ashows the relationship of the optical signal and optical clock accordingto a conventional technology in the case when the wavelengths of bothlights are separated; FIG. 2B shows the case of the present embodimentin which the wavelengths of both lights are in close proximity such thatcoherent interference is observed. In the case of FIG. 2B, i.e., in thecase when the wavelengths λ_(sig) and λ_(clk) of the aforementioned areclose enough such that coherent interference is observed, a componentcorresponding to the correlation of both light is generated in a newwavelength Σ_(FWM) by four-wave mixing between the optical signal andoptical clock within the semiconductor laser (Note: 1/λ_(FWM) =2/λ_(sig)-1/λ_(clk)). The present embodiment utilizes this phenomenon.

The present phenomenon is an ultra-high-speed non-linear opticalphenomenon which is, in essence, completely different from that of gainmodulation according to the conventional art. A more concreteexplanation is contained in Y. R. Shen, "The Principles of NonlinearOptics", Wiley Interscience, pp. 242˜266, 1984. In this manner, thelight generated by means of four-wave mixing includes a correlationcomponent (nΔf) of both entities. Therefore, after extracting thisfour-Wave mixing light by means of an optical band pass filter 104, PLLoperation is achieved as described in the conventional art, i.e., byconverting into an electrical signal by means of optical receivingcircuit 105, comparing a standard signal to an n-multiplied nΔf signalby means of phase comparator 106, followed by feedback of this output toVCO 107.

In addition, with regard to the action of phase comparator 106, it isalso possible to use the circuit construction shown in FIG. 1B. In FIG.1B, the output frequency of optical receiving circuit 105 is multipliedby m/n (m is a rational number) by means of dividing circuit 113, andboth frequencies to be inputted into phase comparator 106 are comparedto the phases of both lights as mΔf by means of multiplying the outputof a.c. signal generator 111 by m. At this time, when m=1, frequencymultiplying circuit 112 can be omitted, and when m=n, dividing circuit113 can be omitted.

Modification Example 1

FIG. 3A is a diagram showing a modification of the aforementioned firstembodiment. In this Figure, an optical signal input terminal 301,optical coupler 302, optical fiber 303, optical bandpass filter 304,optical receiving circuit 305, phase comparator 306, VCO 307, micro-wavemixer 308, optical pulse generator 309, optical pulse multiplexer 310,low frequency oscillator 311, and frequency multiplier 312 are provided.

In the present Modification Example, optical fiber 303 is used as themedium for generating four-wave mixing. In this optical fiber, therespective wavelengths of the optical signal and optical clock may beset to within ±10 nm of the zero dispersion wavelength of optical fiber303.

In addition, as shown in the first embodiment, it is also possible touse the structure shown in FIG. 3B with regard to the presentmodification. The actions are identical to those described in the caseof FIG. 1B.

Modification Example 2

FIG. 4A is a diagram showing another modification of the firstembodiment. In this Figure, optical signal input terminal 401, opticalcoupler 402, non-linear optical crystal 403, optical bandpass filter404, optical receiving circuit 405, phase comparator 406, VCO 407,micro-wave mixer 408, optical pulse generator 409, optical pulsemultiplexer 410, low frequency oscillator 411, and frequency multiplier412 are provided.

In the present Modification Example, the sum-frequency generationphenomenon in non-linear optical crystal is used as the opticalmodulating means. This sum frequency generation phenomenon outputs, atthe time when two types of light of optical wavelengths ν1 and ν2 areincident into the nonlinear optical crystal, a light of an opticalfrequency of the sum of both lights at a magnitude in proportion to theproduct of the intensities of the incident two lights. Details of thisphenomenon can be found by referencing H. Takara, et al.,("Ultra-high-speed optical waveform measurement method using opticalsampling with sum-frequency generation (in Japanese), Denshi JouhouTsushin Gakkai Ronbunshi B-1, J75-B-1; pp. 372-380: 1992). With regardto this phenomenon, the optical frequency of the correlation signalgenerated becomes (ν1+ν2), e.g., in the case when two lights measuring1.55 μm and 1.3 μm, results in a sum frequency light of 0.7 μm. In thecase of four-wave mixing described in the aforementioned first andsecond embodiments, it is necessary for the wavelengths of the incidenttwo lights to lie within approximately 10 nm of each other, and thewavelength of the output light is also at position within approximately10 nm of the incident lights. In contrast, in the sum-frequencygeneration, it is possible to achieve this sum-frequency generation overa wider wavelength range than in the case of four-wave mixing, by meansof adjusting the angles of incidence and the like of the lights into thecrystal, e.g., as shown by the lights of 1.55 μm and 1.3 μm. As thenon-linear optical crystal, any material capable of achievingsum-frequency generation, such as LiIO₃, LiNbO₃, KTP, KNbO₃, and thelike can be used as mentioned in the above-referenced document. Sincethe sum-frequency light generated possesses the exact same correlationcomponent as the four-wave mixing light, PLL can be achieved as in thefirst and second embodiments by converting the aforementioned into anelectrical signal and then conducting feedback to the VCO.

In addition, as shown in the first embodiment, it is also possible touse the structure shown in FIG. 4B with regard to the presentmodification. The actions are identical to those described in the caseof FIG. 1B.

Modification Example 3

FIG. 5A is a diagram showing another modification of the firstembodiment. In this Figure, voltage-controlled oscillator 501, mixer502, optical pulse generator 503, optical coupler 504, optical fiber505, optical coupler 506, optical splitter 507, optical signal inputport 508, optical output port 509, optical receiving circuit 510, lowfrequency oscillator 511, phase comparator 512, optical pulsemultiplexer 513, and frequency multiplying circuit 514 are provided.

In the following, the actions of the present invention will be explainedin accordance with the present modification. Initially, the opticalsignal pulse series is inputted into optical coupler 506 from inputterminal 508. With regard to optical coupler 506, the branching ratio ofthe light intensity is set to 1:1. The optical signal inputted frominput terminal 508 is then divided into 2 by means of optical coupler506, and after propagating over the same route in both directions, theoptical signal returns to optical coupler 506 in the same phase, andthen exits from incident port 508. When an optical control pulse entersvia optical coupler 504, the control light propagates only in theclockwise direction of the Figure over a loop constructed by means ofoptical fiber 505. Hence, the optical signal propagating reciprocally inthe inverse direction over the same loop undergoes a phase shift of adifferent quantity by means of a non-linear optical effect (optical Kerreffect) from the control light. As a result, when the aforementionedreturns to optical coupler 506, the phase balance of both lights isdestroyed. A portion of the signal light then exits through another portof optical coupler 506 in response to this phase difference, passesthrough optical splitter 507, and is outputted from output port 509. Inother words, calculation of the product of the optical signal pulse andoptical control pulse is conducted by means of the loop comprisingoptical fiber 505, optical coupler 504, and optical coupler 506.

The output optical waveform at the time when both of the aforementionedlights enter the optical non-linear loop mirror formed from coupler 504,coupler 506, and fiber 505 is provided by means of the product of theintensities of both lights. In the output light from this loop mirror, arepetition frequency as described in the aforementioned first embodimentis generated as the nΔf component, and thus PLL operation can beachieved according to the same operational principles as in the firstembodiment. In the present embodiment, ultra-high-speed optical PLL canbe achieved due to the use of ultra-high-speed optical Kerr effect asthe optical modulating means.

In addition, as shown in the first embodiment, it is also possible touse the structure shown in FIG. 5B with regard to the presentmodification. The actions are identical to those described in the caseof FIG. 1B.

Second Embodiment

FIG. 6A is a diagram showing a structure according to the secondembodiment of the present invention. In this Figure, optical signalinput terminal 151, optical coupler 152, traveling-wave semiconductorlaser amplifier 153, optical band pass filter 154, optical receivingcircuit 155, phase comparator 156, voltage-controlled oscillator 157(VCO), micro-wave mixer 158, optical pulse generator 159, low frequencyoscillator 160, and frequency multiplier 161 are provided. Theoscillation frequency f₀ of VCO 157 is set such that the bit rate of theoptical signal inputted from optical signal input terminal 151 becomesnf₀ (n is an integer of at least 1).

In the following, the operation of the present embodiment will beexplained. The output signal of VCO 157 undergoes a frequency shift bymeans of low frequency oscillator 160 and micro-wave mixer 158, and thendrives optical pulse generator 159. The repetition frequency thengenerates an optical clock pulse of f₀ +Δf (or f₀ -Δf, or f₀ ±Δf). Asthe waveform of the optical clock pulse in the present invention, it isnecessary to include a harmonics component in order to cope with thenarrow pulse width, rather than using a sine wave. Assuming that thetime waveform Pc(t) of the optical pulse series generated isGaussian-shaped, then the following formula expresses this time waveformPc(t).

    PC (t)=ΣAexp {-α (t+kT).sup.2}                 (6)

In the above formula, Σ is an operator signifying the overall sum withrespect to k=-∞˜+∞. In addition, A and α represent constants, andT=1/(f₀ +Δf). When expanding Pc (t) into a Fourier series, the followingformula results.

    Pc (t)=a  1+2Σexp {-(1/α) (nπ/T).sup.2 }·cos {n·2π(f.sub.0 +Δf) t}!                  (7)

In the above equation, Σ is an operator signifying the overall sum withrespect to n=1˜+∞. In Formula (7), an n× harmonics component n (f₀ +Δf)exists in the second item. This nth harmonics component decreases withincreasing n due to the reduction of the coefficient. However, when thepulse width is narrow (i.e., α is small), the coefficient becomes large,and thus it is possible to generate the nth harmonics.

In the present embodiment, since the nΔf component is generated by meansof detecting the correlation between this nth harmonics n(f₀ +Δf) andoptical signal component, the optical time division multiplexing circuitshown in FIGS. 11 and 12 is not necessary. In order to generate acorrelation signal of a sufficient level with respect to an opticalsignal of a high bit rate, it is necessary for the clock pulse producedto possess a narrow pulse width. At present, use of a gain switchedsemiconductor laser, mode locked laser, or the like as the ultra-shortoptical pulse light source, allows the production of an optical pulsewith a pulse width of 5 ps or less: Using this optical pulse, it ispossible to conduct correlation detection even with regard to an opticalsignal of 100 Gbit/s or more. FIG. 7 shows a spectrum of an opticalsignal and clock pulse inputted into optical coupler 152, and a spectrumof the optical output of traveling wave semiconductor laser amplifier153.

The operation of traveling wave semiconductor laser amplifier 153 whichserves as the optical correlation detecting means is identical to thethat of the structure described in FIGS. 1, 2A, and 2B of the firstembodiment. In the case when wavelength λ_(sig) of the optical signaland wavelength λ_(clk) of the optical clock entering into traveling wavesemiconductor laser amplifier 153 are close enough to coherentlyinterfere, a component corresponding to the correlation of both lightsis generated at a new wavelength λ_(FWM) by means of four-wave mixinggenerated between the optical signal and optical clock within thesemiconductor laser.

The light generated either by an optical signal undergoing gainmodulation or by means of four-wave mixing includes the correlationcomponent of both lights (hΔf). Therefore, after extracting theaforementioned optical signal or four-wave mixing light by means ofoptical bandpass filter 154, PLL operation can be achieved in the samemanner as the conventional art by converting either of theaforementioned into an electrical signal using optical receiving circuit155, and with regard to the nΔf component therein, comparing a standardsignal with n-multiplied nΔf signal by means of phase comparator 156,and then conducting feedback of this output to VCO.

In addition, with regard to the action of phase comparator 156, it isalso possible to use the circuit construction shown in FIG. 6B. In FIG.6B, the output frequency of optical receiving circuit 155 is multipliedby m/n (m is a rational number) by means of dividing circuit 162, andboth frequencies to be inputted into phase comparator 156 are comparedto the phases of both lights as mΔf by means of multiplying the outputof a.c. signal generator 160 by m. At this time, when m=1, frequencymultiplying circuit 161 can be omitted, and when m=n, dividing circuit162 can be omitted.

Modification Example 4

FIG. 8A is a diagram showing a modification of the second embodiment. Inthis Figure, optical signal input terminal 351, optical coupler 352,optical fiber 353, optical bandpass filter 354, optical receivingcircuit 355, phase comparator 356, voltage-controlled oscillator 357(VCO), micro-wave mixer 358, optical pulse generator 359, low frequencyoscillator 360, and frequency multiplier 361 are provided.

In this present modification, optical fiber 353 is used as the mediumfor generating four-wave mixing. In the case of the presentmodification, it is possible to satisfy the phase matching conditions bymeans of setting the respective wavelengths of the optical signal andoptical clock to within ±10 nm of the zero-dispersion wavelength ofoptical fiber 353. In this manner, four-wave mixing can then begenerated in the same manner as in the traveling wave semiconductorlaser amplifier, and hence PLL is subsequently achieved by means of thepresent structure.

In addition, as shown in the second embodiment, it is also possible touse the structure shown in FIG. 8B with regard to the presentmodification. The actions are identical to those described in the caseof FIG. 6B.

Modification Example 5

FIG. 9A is a diagram showing another modification of the secondembodiment. In this Figure, voltage-controlled oscillator 551, mixer552, optical pulse generator 553, optical coupler 554, optical fiber555, optical coupler 556, optical splitter 557, optical signal inputport 558, optical output port 559, optical receiving circuit 560, lowfrequency oscillator 561, phase comparator 562, and frequencymultiplying circuit 563 are provided.

In this present modification, since an optical clock pulse incorporatingthe nth-higher harmonic wave component light is generated from pulsegenerator 553 as in the second embodiment, an optical time divisionmultiplexing circuit is unnecessary. With the exception of thisaforementioned point, the remaining operation is conducted in the samemanner as in Modification Example 3.

In other words, the optical output waveform at the time when the opticalsignal pulse and optical control pulse enter an optical non-linear loopmirror formed by means of couplers 554 and 556 and fiber 555, isprovided by means of the product of the light intensities of bothlights. In the same manner as in the first embodiment, in the opticaloutput from this aforementioned loop mirror, the nΔf component isgenerated by the repetition. frequency, and thus, PLL operation isachieved by means of the same operational principle as in the firstembodiment. In the present modification, ultra-high-speed optical Kerreffect is used as the optical correlation detecting means, and thus therealization of ultra-high-speed optical PLL can be anticipated.

In addition, as shown in the second embodiment, it is also possible touse the structure shown in FIG. 9B with regard to the presentmodification. The actions are identical to those described in the caseof FIG. 6B.

Modification Example 6

FIG. 13 is a diagram showing another modification of the secondembodiment. In this Figure, optical signal input terminal 901, opticalcoupler 902, non-linear optical crystal 903, optical bandpass filter904, optical receiving circuit 905, phase comparator 906, VCO 907,microwave mixer 908, optical pulse generator 909, low frequencyoscillator 910, frequency multiplier 911, and dividing circuit 912 areprovided.

In the present Modification Example, the optical sum-frequencygeneration phenomenon of a non-linear optical crystal described inModification Example 2 is used as the optical modulating means. Theactions are identical to those described in the case of FIG. 6B.

What is claimed is:
 1. Optical correlation detecting circuitcomprising:optical combining means for combining an optical signal pulseand an optical clock pulse possessing a sufficiently narrow pulse widthincluding nth harmonics, n being a natural number, component of arepetition frequency; optical non-linear loop mirror means foroutputting a product of intensities of said optical signal pulse andsaid optical clock pulse; and optical splitting means for extractingfrom an optical output signal of said optical non-linear loop mirrormeans only a correlation component of said optical signal pulse and saidoptical clock pulse.
 2. Optical correlation detecting circuit as recitedin claim 1, wherein said optical signal pulse and said optical clockpulse are lights of wavelengths that coherently interfere with eachother.
 3. Optical correlation detecting circuit as recited in claim 1,wherein said optical signal pulse and said optical clock pulse arelights of wavelengths that coherently interfere with each other; and acorrelation component of a coherent light is generated.
 4. Opticalcorrelation detecting circuit as recited in claim 1, wherein a gain withrespect to an optical signal pulse of said optical non-linear loopmirror means is modulated using the intensity of an optical clock; andsaid correlation component being an optical signal undergoing intensitymodulation.
 5. Optical correlation detecting circuit as recited in claim1, wherein said correlation component of said optical signal pulse andsaid optical clock pulse is a four-wave mixing light generated in saidoptical non-linear loop mirror means.
 6. Optical correlation detectingcircuit comprising:optical combining means for combining an opticalsignal pulse and an optical clock pulse possessing a sufficiently narrowpulse width including nth harmonics, n being a natural number, componentof a repetition frequency; optical fiber for outputting a product ofintensities of said optical signal pulse and said optical clock pulse;and optical splitting means for extracting from an optical output signalof said optical fiber only a correlation component of said opticalsignal pulse and said optical clock pulse; wherein, said optical signaland said optical clock respectively possess wavelengths in a vicinity ofa zero dispersion wavelength of said optical fiber.
 7. Opticalcorrelation detecting circuit comprising:optical combining means forcombining an optical signal pulse and an optical clock pulse possessinga sufficiently narrow pulse width including nth harmonics, n being anatural number, component of a repetition frequency; optical fiber foroutputting a product of intensities of said optical signal pulse andsaid optical clock pulse; and optical splitting means for extractingfrom an optical output signal of said optical fiber only a correlationcomponent of said optical signal pulse and said optical clock pulse;wherein a loop is formed by optically coupling said optical combiningmeans, said optical fiber, and said optical splitting mean, and whereinsaid optical combining means separately includes a means for receivingsaid optical signal pulse and means for receiving said optical clockpulse.